Shouldn't all the instances show the same community content?

I'm learning about the Fediverse and am confused about how federation is supposed to work. I understand that there can be communities with the same name in different instances, with different content. But I also understand you can subscribe to another instance's community. For example, there are sysadmin commnunities at lemmy.world, lemmy.one, and beehaw.org (among others). If we focus on one specific community, let's say sysadmin@lemmy.world, we can find that community from any of the instances. If I go to each instance and look at sysadmin@lemmy.world from each one, I can see the same pinned post is at the top of each one instance's view ("Calling all /r/sysadmin reddit refugees!" by DarraignTheSane).

Great!

However, if I look at that pinned thread from each of the three instances, the comment stream is different. The post itself is the same, but the comment thread is a mixed bag. Some comments seem to appear in multiple instances while others only in one or two, but never all three

lemmy.world shows 11 comments lemmy.one shows 6 comments beehaw.org shows 4 comments

On lemmy.world, the second newest comment says "Nice! It feels like home." This comment also shows up on lemmy.one however not on beehaw

The newest comment on lemmy.world says "yeeey" but doesn't appear in any other instance's view of sysadmin@lemmy.world

This is just one specific example. Are you not supposed to get the same content, when looking at the same community, regardless of what instance you are logged into when viewing it? Or am I missing something?